As the population of cancer, transplantation, abdominal surgery, and other immunocompromised patients continues to grow, there is a concomitant increase in the number of patients needing treatment for systemic fungal infections. Traditionally, systemic mycoses antibiotics are dominated by just three classes of drugs, polyenes, most notably Amphotericin B and Nystatin; azoles, such as Flucanazole, Itraconazole, Ketoconazole, and Voriconazole; and echinocandins, such as Caspofungin, Micafungin, and Anidulafungin. Each of these drug classes possess significant limitations in terms of efficacy, toxicity, drug-drug interactions, and the generation of resistant organisms (e.g. Barrett, 2002; Fishman, 2002; Girmenia and Martino, 2003; Gupta and Thomas, 2003; Park et al., 2005; Pavie et al., 2005; Balashov et al., 2006; Perlin et al., 2007; Choi et al., 2008). Consequently, there is an urgent need for new drugs with novel modes of action to treat of systemic mycoses.
The Aureobasidium pullulans strain BP-1938 produces a 9-amino acid cyclic peptide, referred to as Aureobasidin A (“AbA”). This compound is a potent, fungicidal drug that is very well tolerated in animals and humans (Takesako et al., 1993). AbA also has a unique mode of action that targets inositol phosphorylceramide (“IPC”) synthase; an enzyme in the fungal sphingolipid biosynthesis pathway. Attempts to develop spontaneous resistance mutants to AbA has, to date, been unsuccessful, suggesting that resistance development in clinical settings with this compound will be very slow. Resistance mutants can be generated by chemical mutagenesis; however, the viability of the resulting organism is highly compromised. (Heidler et al., 1995; Hashida-Okado et al., 1996). Unfortunately, native AbA does not have a perfect target spectrum: it is very efficacious against virtually all Candida species, including C. albicans. It is also efficacious against most Cryptococcus species, including C. neoformans. However, it shows little activity towards most Aspergilli, and most notably A. fumigatus. (Takesako et al., (1993) J. Antibiot. 46, 1414-1420). Since Candida and Aspergillus are the two most common human pathogens and broad-spectrum antibiotics are preferred in the clinic, AbA's lack of efficacy against aspergilli has hampered its development into a marketed drug (Takesako et al., 1993). The reason for A. fumigatus′ resistance to AbA is not that the target enzyme, inositol phosphorylceramide (IPC) synthase in A. fumigatus is resistant to the compound, but rather that this organism has one or more pumps capable of efficiently clearing the drug (Ogawa et al., 1998; Zhong et al., 2000). Thus, the development of AbA derivative(s) capable of avoiding or blocking the A. fumigatus pumps would greatly enhance the development potential and marketability of the compound.
A small number of AbA derivatives have been prepared by synthetic chemistry (reviewed in Kurome and Takesako, 2000) and evaluation of these compounds has demonstrated that AbA's pharmacological properties can be altered significantly by modifying and/or exchanging amino acids in the sequence. Most importantly, AbA derivatives have been generated that appear to have similar antifungal activity against A. fumigatus and C. albicans (Kurome and Takesako, 2000). Specifically, substitution of the N-methyl-L-phenylalanine residue at position 4 with a N-methyl-D-alanine or a sarcosine residue results in a compound with significant activity against A. fumigatus; and combining this substitution with substitution of the L-phenylalanine residue at position 3 with derivatized L-tyrosine, phenylalanine or alanine residues, results in compounds with A. fumigatus minimum inhibitory concentrations (MICs) in the single digit microgram/ml range. Importantly, while gaining considerable activity against A. fumigatus, these compounds retain their activity against C. albicans and Cryptococcus neoformans. Nonetheless, the synthetic chemistry approach that was used for the synthesis of these compounds was very complex. The entire synthesis process included 21 steps and the overall yield was less than 1%. In addition, the one or more of the 21 steps constitutes a high risk procedure that is not amenable for industrial production. During the late 1990s and early 2000s, Pharmacia, and later Pfizer, Inc., attempted to improve Takara's 21 step synthesis. However, these attempts were unsuccessful and Pfizer abandoned the project in 2005.